Incipient Melting and Corrosion Properties of Friction Stir Welded AA2024 T3 Joints

Abstract

The effect of welding speed on strength and corrosion behavior of FSW 2024-T3 joints was investigated in the present study. For both strength and corrosion, the transition from the thermo-mechanically affected to the heat affected zone (TMAZ/HAZ) turned out to be the most critical site. Higher welding speeds produced both a smaller hardness drop in the TMAZ/HAZ and a smaller width of the softened zone. Thereby, tensile strength could be improved and the intergranular corrosion attack localized to the overaged TMAZ/HAZ could be reduced. Paradoxically, at lower heat input (i.e. at higher welding speed), the weld nugget region also suffered enhanced intergranular corrosion. SEM examinations of the top surface of FSW 2024-T3 joints revealed the presence of constitutionally melted Al2MgCu (S-phase) particles in that region. The S-phases were mainly localized to the advancing side of the joint, and their volume fraction increased with growing welding speed. Hence it is suggested that a focalization of the thermal source resulted in incipient melting. Eutectic S-phases were formed and served as preferential corrosion sites lowering corrosion resistance of the weld nugget.